1. Field of the Invention
This invention relates to the production of acrylic polymers. More particularly, it relates to the use of a dual peroxide initiator system whereby the acrylic sheet may be made at a high production rate yet yield sheet which has high thermal stability, is bubble free, and has a low level of unreacted monomer.
2. Description of the Prior Art
The manufacture of acrylic polymers such as cast poly(methyl methacrylate) has increased substantially due to growing demand. With increasing use of acrylic polymer, much effort has been made to find faster polymerization systems which will improve production rates. While there has been notable success in finding ways to shorten polymerization times, the quality of the finished product has also deteriorated.
Casting of methyl methacrylate monomer containing some polymer, commonly referred to as prepolymer or prepoly syrup, was for years carried out in cells prepared from plate glass, flexible gaskets and steel clips. This type of process was slow, taking 16-170 hours. Polymerization times were shortened with the development of continuous processes such as those described in U.S. Pat. Nos. 3,872,197 issued Mar. 18, 1975 and 4,046,850 issued Sept. 6, 1977, both to Kato et al. While the residence time in the continuous casting units described in these patents is much shorter than in the cell casting operation, the units are very expensive making it desirable to increase the production rates as much as possible.
A variety of initiators and initiator systems have been suggested and used to increase the production rate of cast poly(methyl methacrylate) sheet. Many have been combinations of a variety of peroxide and azo initiators. In fact, dual initiator systems of different peroxide/peroxide, peroxide/azo, and azo/azo initiators have been tried. A good description of these prior art initiator systems is found in U.S. Pat. No. 4,328,329 issued May 4, 1982, to Ernest R. Novak, which is assigned to the same assignee as the present application. Novak was concerned with finding a combination of initiators which would provide fast polymerization with low residual monomer. To do so, he selected a dual initiator system consisting of an active peroxide and less active azo. The active peroxide (t-butyl or t-pentyl peroxyneodocanoate) insured fast polymerization. The less active azo (2,2'-azobis(isobutyronitrile)) reduced the residual monomer to acceptable levels. Together, these initiators provided a combination which seemed to work well.
In reviewing other prior art initiator systems in his patent, Novak disclosed a dual peroxide described in an article by D. W. Wood, Plastics Engineering, May 1975, pp. 51-53. Wood's article recommended the combined use of t-butyl peroxyneodecanoate and decanoyl peroxide. The last paragraph of the Wood article, however, stated: "The resulting cured castings were hard and glasslike, and exhibited some cavities that could be attributed to shrinkage during polymerization of the material." Acrylic sheet containing cavities is unacceptable. Novak repeated Wood's experiments and confirmed that voids formed in the sheet. He also found that the unconverted monomer was 8.9 and 11.4% in the two experiments he conducted. Since good quality acrylic sheet should have less than 2.5% residual monomer and no voids or cavities. Novak concluded that his peroxide/azo initiator system was superior to the dual peroxide suggested in the Wood article.
Subsequently, it was found that acrylic sheet produced in accordance with the Novak patent was prone to bubble formation when heated. This problem was particularly severe when the acrylic sheet was thermoformed at temperatures at about or above 200.degree. C. since gas solubility decreases with increasing temperature. The bubble formation was aggravated by nitrogen gas which is emitted as the azo initiator decomposes during polymerization. The nitrogen is less soluble in acrylic sheet than such gases as carbon dioxide and will cause bubbles to form at lower concentrations. This problem is less likely to occur with peroxides which emit carbon dioxide when they decompose. The carbon dioxide is more soluble than nitrogen and is less likely to cause bubbling either during polymerization or subsequently during thermoforming of the acrylic sheet. However, the decomposition properties of the peroxides need to be selected carefully since too much carbon dioxide can also cause bubbling and adversely affect thermal stability.
It is desirable to have an initiator system which will provide a fast production rate, produce sheet free of cavities or other defects, relatively free of residual monomer and which has high thermal stability so that it can be subsequently thermoformed at temperatures at or above 200.degree. C. without bubble formation. The peroxide/azo initiators of the Novak patent achieved a fast production rate and low residual monomer, but did not provide sheet with high thermal stability. The dual peroxide system disclosed in the Wood article, as noted by both Wood and Novak, produced sheet with cavities.